Various non-limiting embodiments disclosed herein relate generally to thickening systems including xanthan gum that can be used in aqueous-coating compositions and aqueous coating-compositions prepared using the same. Still other non-limiting embodiments disclosed herein relate to methods of making and using thickening systems including xanthan gum and methods of making and using aqueous-coating compositions including xanthan gum.
Aqueous-coating compositions, such as latex paints, generally comprise a latex polymer (i.e. polymer suspension or emulsion in water), one or more inorganic pigments, and a variety of other components that are selected to modify one or more of the processing, application, and filming-forming characteristics of the aqueous-coating compositions. For example, in addition to a latex polymer and inorganic pigments, typical latex paints generally include at least one dispersant that aids in dispersing the inorganic pigments in the paint composition, at least one thickener that modifies the rheological characteristic of the paint composition, and at least one coalescing aid that aids in film-formation as the paint composition dries. Other common additives to latex paint compositions include, for example, surfactants, solvents, pH modifiers, defoamers, biocides, colorants, and humectants.
One common thickener for aqueous-coating compositions, such as latex paint compositions, is hydroxyethylcellulose (“HEC”). HEC modifies the viscosity and rheology of the aqueous-coating composition by imparting thrixotropic (or shear-thinning) behavior to the coating composition. More specifically, when added to an aqueous-coating composition, an increase in the hydrodynamic volume (swelling) of the HEC polymer chains results from hydrogen bonding between the HEC and the water in the coating composition. Swelling and entanglement of the HEC polymer chains causes the viscosity of the coating composition to increase. However, when a shear force is applied to the aqueous-coating composition, for example when the coating composition is applied to a substrate, the viscosity of the HEC thickened aqueous-coating composition decreases, permitting good flow of the coating onto the substrate. When the shear force is removed, the viscosity again increases, thereby reducing the tendency of the coating to run.
Hydrophobically modified hydroxyethylcellulose (“HMHEC”) is also a known thickener for aqueous-coating compositions. Like HEC, HMHEC modifies the viscosity and rheology of the aqueous-coating composition by imparting thrixotropic (or shear-thinning) behavior to the coating composition. Further, like HEC, HMHEC can increase the viscosity of the coating composition due to interaction with the water in the coating composition. However, unlike HEC, because HMHEC contains hydrophobic groups (for example, long-chain alkyl groups) bound to the HEC polymer chain, HMHEC also builds viscosity due to association of the hydrophobic groups.
Xanthan gum is a high molecular weight, naturally occurring polysaccharide produced by the fermentation of glucose with Xanthomonia campestris. Like HEC and HMHEC discussed above, xanthan gum can be used as a thickener to impart thrixotropic behavior to aqueous-coating compositions. However, unlike HEC and HMHEC, when incorporated into water, xanthan gum molecules have a stiff, rod-like structure. Thus, rather than building viscosity by polymer chain entanglement and/or hydrophobic associations, xanthan gum is generally believed to build viscosity in aqueous-coating compositions by the formation of a three-dimensional network of xanthan gum molecules held together by hydrogen bonds. Since this network structure can be rapidly broken down by the application of an external shear force to the structure, aqueous-coating compositions thickened by xanthan gum are highly shear-thinning. Further, since the viscosity-building network structure of hydrated xanthan gum is rapidly re-established when the external shear force is removed, aqueous-coating compositions thickened with xanthan gum tend to regain viscosity more rapidly than coating compositions thickened with HEC or HMHEC.
However, due to the rapid hydration of unmodified xanthan gum in water, direct incorporation of unmodified xanthan gum into aqueous-coating compositions can be difficult. For example, directly adding unmodified xanthan gum powder to an aqueous-coating composition can result in an extremely rapid increase in the viscosity of the aqueous-coating composition and the formation of a gel containing agglomerates or lumps of unhydrated xanthan gum. Such gel formation is generally undesirable as it can make both mixing of the coating composition and incorporation of other components into the coating composition difficult. Thus, while both HEC and HMHEC can be added directly to an aqueous-coating composition in the form of a powder, attempts to add xanthan gum powders directly to aqueous-coating compositions have generally involved the use of specialized mixing procedures or equipment, for example high-shear mixers, or xanthan gum powders that have been encapsulated or surface-modified with another substance to retard hydration.
Although it is possible to premix the aforementioned thickeners with a solvent such as propylene glycol or a coalescing aid such as an ester alcohol prior to addition of the thickener to the coating composition, the use of these solvents and coalescing aids is generally undesirable in aqueous-coating compositions due to their high-VOC (volatile organic compound) content. Further, while premixes of HEC and HMEC with water can be made that do not contribute to the VOC content of the coating composition, as discussed above, it is generally difficult to incorporate xanthan gum directly into water unless specialized mixing or surface-modified powders are used. Additionally, since water premixes do not aid in film-formation during drying of the coating composition, even where such water-based premixes are employed they do not reduce or eliminate the need to add a coalescing aids to the coating composition.
Accordingly, it would be useful to develop aqueous-coating compositions that include a xanthan gum thickener and a coalescing aid that does not contribute to the VOC content of the coating composition, as well as xanthan gum-based thickening systems that could be readily incorporated into a variety of aqueous-coating compositions and that would not contribute to the VOC content of aqueous-coating compositions into which they were incorporated. Further, it would be desirable if such thickening systems could also reduce or eliminate the need to use other coalescing aids in the coating composition that contribute to the VOC-content of the aqueous-coating composition.